The present invention relates to EEPROM memory cell array architectures such as are used, for example, in programmable logic devices (PLDs) such as complex programmable logic devices (CPLDs). More particularly, the present invention concerns a memory cell array architecture that substantially eliminates leakage current to allow for reading memory cells in a memory cell array of, for example, a CPLD at lower voltages than are possible with prior art architectures, thereby facilitating development of low voltage applications.
Referring to FIG. 1, a conventional electrically erasable programmable read-only memory (EEPROM) cell 20 is shown which is commonly used to implement embedded non-volatile memory circuitry in a CPLD, wherein the EEPROM cell 20 serves as a memory cell in an array of memory cells operable to store a designed configuration. As illustrated, each such EEPROM cell 20 broadly comprises a bitline read (BLrd) node 22; a bitline program (BLpr) node 24; a bitline ground (BLgrnd) node 26; an access gate (AG) node 28; an AG program transistor 30; an AG read transistor 32; a floating gate (FG) memory transistor 34; and a control gate (CG) node 36.
Referring also to FIG. 2, a prior art memory cell array architecture 40 is shown wherein a plurality of the EEPROM cells 20 are connected to bitlines 42 of programming paths and read paths. The BLrd nodes 22 of all of the EEPROM cells 20 in each bitline 42 are connected to a sense amplifier (sense-amp) 46. The BLgrnd nodes 26 of all of the EEPROM cells 20 in the bitline 42 are connected together and to a common ground. The result is that all leakage currents from unselected EEPROM cells 20 are added together along bitline 42. In order to keep the total leakage current sufficiently low, so as not to trip the sense amplifier 46, the threshold voltage (Vt) of read access gate transistor 32 needs to be sufficiently high, or about 0.8V. Consequently, in order to reliably read a selected cell, this requires that the gate voltage on access gate node 28, or Vdd, to be sufficiently high. The power supply Vdd would therefore need to be such that Vgxe2x88x92Vt=Vddxe2x88x92Vt=1.0V, or Vdd=0.8+1.0=1.8V.
Current trends toward lower Vdd in integrated circuit electronics pose new challenges to circuit implementation. One problem that has arisen, for example, is that threshold voltages (Vt) in CMOS transistors, such as, for example, the EEPROM cell AG read transistor 32, cannot fall below a certain lower limit without giving rise to undesirable off-state leakage currents. This limitation is encountered when reading the EEPROM cells 20 using the sense-amp 46.
In the prior architecture 40, the read path bitlines 42, in which the AG transistor 32 of each EEPROM cell 20 is connected in series with its FG memory transistor 34, are connected in parallel. The sense-amp 46 triggers at a bitline current of approximately 6 xcexcA. When Vdd=1.8V and Vt=0.8V for the AG read transistor 32, gate voltage (Vg)xe2x88x92Vt=1.0V. With this drive voltage, the EEPROM cells 20 will deliver sufficient read current, approximately 15 xcexcA, to reliably trigger the sense-amp 46. Maximum allowable leakage current from a non-selected EEPROM cell 20, however, can be no more than the total bitline leakage current, which is less than approximately 1 xcexcA, so that the leakage current doesn""t trigger the sense-amp 46. Because each bitline 42 may include, for example, 100 EEPROM cells 20 connected in parallel, the maximum leakage current per EEPROM cell 20 must be less than 10 nA.
Furthermore, because reading of the memory cell array is triggered on power-up of the CPLD, the Vdd at the time of power-on reset (POR) will be lower than the target Vdd by approximately 0.4V, making Vdd=1.4V at power-up. Therefore, the maximum allowable read current for a selected programmed (low Vt) EEPROM cell 20 is
Iread greater than xcx9c10 xcexcA when Vdd=1.4V.
At the same time, the EEPROM cell 20 must not exceed the maximum allowable leakage current for an unselected EEPROM cell 20, which, as mentioned, is
maximum Vdd=1.9V so Ioff less than 10 nA.
It is possible to accomplish this with an AG read transistor 32 having a Vt of 0.8V. Unfortunately, lowering the Vt increases the leakage current by approximately one order of magnitude per 0.1Vt shift, making it practically impossible to lower the Vt of the AG read transistor 32 below 0.8V without risking a read failure due to the EEPROM cell read path bitline leakage current. Thus, with prior art architectures, it is not possible to meet read reliability requirements as Vdd is lowered below 1.8V.
Due to the above-identified and other problems and disadvantages in the art, there exists a distinct need for an improved memory cell array architecture.
The present invention solves the above-described and other problems and disadvantages in the prior art to provide a memory cell array architecture that substantially eliminates leakage current to allow for reading memory cells in a memory cell array at lower voltages than are possible with prior art architectures, thereby advantageously facilitating development of low voltage applications, particularly hand-held low voltage battery-powered devices. The architecture may be used, for example, to implement embedded non-volatile memory circuitry in a PIC device such as a CPLD, wherein the memory cells are conventional EEPROM cells.
In the architecture of the present invention, all of the BLgrnd nodes of the EEPROM cells in the same wordline are connected together in a common BLgrnd line, and each common BLgrnd line is connected through a select transistor to ground. In one embodiment, the select transistor is driven by the same high voltage wordline (HV WL) signal used to select the AG read transistor of each EEPROM cell in the wordline. This results in all unselected EEPROM cells in each bitline having floating BLgrnd nodes, thereby eliminating the off-state leakage current contribution from unselected EEPROM cells. The Vt of the AG read transistor can then be reduced from 0.8V to a significantly lower value, such as, for example, between approximately 0.4V and 0.5V, thereby allowing the EEPROM cell to be successfully read at a correspondingly lower Vdd voltage. Furthermore, since the access gate read transistor 32 and the access gate programming transistor 30 typically have the same Vt""s, the lower Vt of the AG programming transistor 30 results in a lowered voltage drop across the AG programming transistor, which results in a corresponding improvement in the programming efficiency of the cell and a lower programmed Vt of the FG memory transistor 34, in turn leading to a higher read current in a selected cell.
Thus, it will be appreciated that the memory cell array architecture of the present invention provides a number of substantial advantages over prior art architectures, including, for example, that the leakage current contribution from unselected EEPROM cells is advantageously eliminated. Furthermore, the architecture advantageously allows for reducing the Vt of the AG read and programming transistors from 0.8V to a significantly lower value, such as, for example, approximately between 0.4V and 0.5V, thereby allowing the EEPROM cell to be successfully read at a correspondingly lower Vdd voltage. Additionally, the lower Vt of the AG programming transistor of each EEPROM cell allows for improved cell programming because the voltage drops across the AG programming transistor is reduced, which results in a corresponding improvement in the programmed Vt of the FG memory transistor.
These and other important features of the present invention are more fully described in the DETAILED DESCRIPTION below.